Polyolefin reactors are used for the commercial production of a number of polyolefin compounds, such as polyethylene. In such reactors, for example loop-type reactors, the monomer and optional comonomers to be polymerized are mixed with a diluent such as isobutane and subjected to heat and pressure while circulating through the loops of the reactor. To initiate the polymerization process, a catalyst is introduced into the circulating mixture of monomers and diluent. Once polymerization is initiated, an effluent is formed comprising a slurry of particulate polymer solids suspended in the reaction diluent and unreacted monomers.
Many of the catalysts used to initiate polymerization are particulate in form. Often, such catalysts comprise an active catalytic compound coated over an inert carrier particle. Such a catalyst is generally known as a particulate-supported catalyst. While there are many different types of particulate-supported catalysts, one example that is presently used for the production of polyethylene comprises chromium oxide impregnated in a carrier particle of silica, which may have an average diameter ranging anywhere between about 20 micrometers to 150 micrometers.
Such a particulate-supported catalyst, in dry form, is sand-like in its physical properties. Consequently, in order to facilitate the introduction and uniform distribution of such a catalyst into the pressurized reactor, it is often mixed with some amount of a diluent to form a slurry. The slurry may be fed into the reactor one of two ways, i.e. via a dosing feeder or in a more diluted liquid form via a slurry feeder.
When a dosing feeder is used, the particulate catalyst is mixed with a small amount of diluent such as isobutane used as the diluent in the polymerization effluent. Such mixing is performed in what is known as a “mud pot” in the art by merely pouring the sand-like particulate catalyst and a small volume of diluent together without agitation. The particulate catalyst settles to the bottom of the “mud pot” to form a highly packed, viscous, mud-like slurry. The thick catalyst slurry is conveyed to the mud-pot outlet via a gravity feed into a dosing chamber located in a feeder leading to the reactor. The dosing chamber is movable from a loading position where it receives the mud-like catalyst slurry from the mud-pot outlet, to an unloading position within the feeder where it is opened to release a slug of catalyst slurry into a flow of diluent and from then on to a flow of effluent in the reactor.
As disclosed in U.S. Pat. Nos. 4,610,574 and 6,908,971, when a slurry feeder is used, the particulate catalyst is mixed with a larger volume of diluent (such as isobutane used as the diluent in the polymerization effluent) in a separate agitation vessel outside of the reactor to form a more dilute, much less viscous catalyst slurry. The diluted catalyst slurry is continuously stirred in the agitation vessel to prevent the sand-like particulate catalyst from settling out of the diluent. The resulting diluted catalyst slurry is pumped usually by way of a positive displacement type pump into a conduit leading into the reactor.